The `ExtraErrorInfo` framework doesn't support declaring error codes whose extra info class is under a namespace.

For example, this error info declaration:

error_code("AsyncExecutorShutDown", 280, extra="AsyncExecutorShutDownExceptionInfo")

Generates the following declaration in error_codes.h, which fails to compile:

// ErrorExtraInfo subclasses:
...
class txn::AsyncExecutorShutDownExceptionInfo;